VG – Re the OP, I agree completely that you can’t have (can't program for) the ability to override certain txns and still use historical lineups.
I was only referring to txns. OOTP’s real txns module doesn’t care about downstream affects. It only moves players from one team to the next when it’s time - and it doesn’t care about what team a player is currently on when the txn occurs and doesn't care about which players are involved in a trade (which is why programming for this flexibility shouldn’t be a big deal – the txn would just be ignored by the game). And any concern about the downstream ("Butterfly") effects would be the user’s issue to either account for, ignore, or explain away ("crazy one-sided trade" or "free-agent signing" instead of trade). OOTP doesn’t know and doesn’t need to know. It will continue to do whatever the real txn file tells it to do... or won't do...
In the Freddy Lynn example, if I prevent him from going to the Angels, he stays on the BoSox until which time the txn file sends him to the Orioles. However, I could uncheck the box every time Freddy moves to another team, thus keeping him on the Red Sox for his entire career. All other related player moves (those guys involved in his trades, or those acquired in other trades to account for his departure) would either happen, or not happen, depending upon what I as the user do (or don't do). Either way, OOTP will do what it's told. It doesn't have to be coded to think, "Wait. Fred Lynn isn't being traded to the Angels: What do I do about the others involved in that trade? And what are the downstream effects?" Those wouldn't be the program's concerns. It would either move players to their new team, or it wouldn't if those moves are overridden. Just like it happens occasionally in my game now (via manual editing), only it would be simpler for the user to control.